1. Field of the Invention
This invention relates generally to blown film extrusion lines, and specifically to improved control and blower systems for use with blown film systems.
2. Description of the Prior Art
Blown film extrusion lines are used to manufacture plastic bags and similar products. A molten tube of plastic is extruded from an annular die and then stretched and expanded to a larger diameter and a reduced thickness by the action of overhead nip rollers and internal air pressure. Typically, the annular die or the overhead nip rollers are slowly rotated to distribute film thickness irregularities caused by die imperfections. To control the circumference of the finished tube, it is generally necessary to adjust the volume of air captured inside the tube between the annular die and the overhead nip rollers. It has been conventional to adjust the entrapped volume of air by operating a rotary valve which controls air flow to the annular die, although some control can be obtained by adjusting the supply and exhaust valve and blower systems.
One significant problem with rotary valve mechanisms is that they operate at their best only over a narrow range of loading conditions. More specifically, rotary valves work best when the air pressure load on the rotary valve is matched to the air pressure load of the annular die. When these loads are mismatched, start up operations are difficult, and when the bubble has been started it may be quite unstable, and can be characterized as "shaky". Furthermore, when the loads are mismatched between the rotary valve and the annular die, control over the bubble is impaired; for example, control over an extruded tube may drop from plus or minus one-eighth of an inch in diameter to approximately plus or minus one inch in diameter, an eight fold decrease in control over the extruded tube.
Measurement and control of the extruded film tube circumference is rather important. Mechanical, optical, and acoustic mechanisms have been employed to provide a signal corresponding to the extruded film tube circumference.
Systems which employ mechanical feelers are currently disfavored, since feelers produce deformations in the film which impair the quality and grade of the plastic products. In addition, with mechanical feelers, tube size measurements must be made beyond the molten region of the tube to avoid serious deformations in the tube wall as a result of contact by the feeler. Making the measurement away from the molten region can introduce a detrimental delay into the control system, and reduce accuracy.
Optical and acoustic systems have been provided as substitutes for the mechanical feeler arm. These optical and acoustic systems eliminate the problem of mechanically induced deformations in the extruded plastic tube, but they are more susceptible to false readings than the mechanical systems. Such false readings can occur as a result of the constant flutter of the extruded film tube. For acoustical systems, scattered interrogating pulses, as well as ambient noise, and ambient temperature changes can result in inaccurate readings.
Consequently, most prior art systems use multiple sensors, which are expensive, to reduce the frequency of misreadings. A false reading can result in an unnecessary overinflation or deflation of the extruded film tube, and can result in an exploded or collapsed extruded film tube.
In this worst case situation, the production line is brought to a complete standstill. Such an error can be expensive, since production time is frequently valued at over one thousand dollars per hour. When an extruded film tube is collapsed or damaged by being overblown, a new bubble must be initiated. In the prior art systems, a skilled operator must take control of the system at startup to initiate an extruded film tube.